(Found this thread with a "search").
Why no more answers? I wanted to open a thread with just the same question:
Well, Catalina is no more a beta. Not even a ".0" version.
We have lots of "small" problems, nobody seems to really understand new disk structure, we're prone to lose 32bit compatibility, no more fiddling into system, e-gpu's fail, Time Machine could go wrong, etc.
Please, somebody!: ANY PERFORMANCE GAIN????
In my understanding, having measurable performance gain by upgrading the OS is rare. Same hardware, same max / potential performance.
For upgradable Mac (e.g. Mac Pro 5,1, or the coming 7,1), upgrade the OS means "can use some newer hardware" (e.g. RX 5700XT can only works in Catalina, but not Mojave), therefore, better performance (measurable).
So, unless Mojave has some serious performance bug for your hardware (e.g. driver issue), otherwise, Catalina shouldn't bring any noticeable performance improvement to your Mac.
However, it is possible to have snappier experience even no measurable performance gain. e.g. the newer OS can have better coding for finder, or handling the OS UI, etc. For example, the older OS need 500 line of codes to handle "file coping", but the newer OS only need 400 line of codes to handle the same input. Then, end up same hardware, same driver performance, but still can have 20% improvement in handling the user input. So, the user may able to feel that "the UI for file copying" pop up quicker, in other words, feel snappier (but the actual copying speed most likely still the same, because same hardware, same speed. So, disk speed test won't shows any improvement).